1. Field of the Invention
The present invention relates to the technical field of an optical filter device, in particular, to an optical fiber filter device and a method for manufacturing the same.
2. Description of the Related Art
With rapid development and commercialization of Fiber to the home (FTTH) and Fiber to the X (FTTx), the demand for the real-time monitoring of the optical fiber links at a local end and at a user end has been greatly increased recently. In the current FTTx monitoring system, the links are monitored by an Optical Time Domain Reflector (OTDR). In a dominant TDM-PON configuration, as an Optical Power Splitter is shared by a plurality of users, OTDR can attain results synthesized from the links of the plurality of users instead of the link of a single user. However, in a specific monitoring circumstance, it is necessary to acquire distinct detected images of the links between each user end and the local end.
At present, one solution that has been proposed is to insert a Fiber Brag Grating (FBG) in the user end. The FBG has a very narrow intrinsic band in the reflective spectrum (˜0.5 nm@3 dB). However, the OTDR monitoring system in the FTTx system requires a wider band in the reflective spectrum to be adapted for the deviation or drift (20 nm) of the center wavelength of a laser. Although the technology of chirp can enhance the band width of the FBG, it will be very expensive and the overall optical performance of the monitoring system will be reduced significantly due to the widen band of the FBG.
Conventionally, the FBG may be manufactured by means of a two-beam interferometer or phase masks. The solution employing two-beam interference is generally used in the field of research and development and is not suitable for mass production. In contrast, the phase masks can be used in mass production by virtue of its simplified and controllable process. In consideration of the narrow intrinsic band of the FBG (˜0.5 nm@3 dB) made by single phase mask, a plurality of phase masks or nonperiodic phase masks are necessarily used to form chirp FBG to enhance the band width of reflective spectrum. However, the customized phase mask with the reflective spectrum of 20 nm is extremely expensive. In the meanwhile, in the solution of chirp FBG, the overall optical performance of the system degrades remarkably as the band width of the reflective spectrum of FBG increases.
Therefore, it is desirable to provide an optical filter device with wide reflective spectrum, high reflection isolation and low cost to acquire the required detection signals from the OTDR.